The present invention concerns a finger joint prosthesis generally including a joint body having first and second joint elements. The joint elements are preferably connected together pivotably about an axis of rotation, and each joint element preferably includes an elongate anchoring bar for implementing a secondary securing to a proximal and/or distal phalanx. In addition, one or more eyes having openings for receiving fixing screws for implementing a primary fixing can also be provided on the anchoring bars. The fixing screws provide for primary fixing for fixing the prosthesis in the target position on the bones. The finger joint prosthesis then grows to the bone in the context of the biological or secondary fixing in the bone. Secondary fixing is then effected by way of the surface of the finger joint prosthesis.
Functional disorders of finger middle joints can occur on the one hand by virtue of an inherited degenerative illness, joint arthrosis. In that case, the illness leads to “wear” of the joint cartilage and consequently corresponding inappropriate stresses with complete changes in the joint surfaces which result in painful limitation or even seizure of the joint function. On the other hand, functional disorders in the finger middle joint can also occur because of injury, for example, due to luxation of the joint or due to a joint fracture. Primary treatment of a joint fracture generally leads to a so-called “post-traumatic arthrosis” of the joint, which also leads to a painful functional limitation or functional immobilization of the finger middle joint.
One way of eliminating the above-indicated functional disorders is to stiffen the joint in a position which is favorable to its function. That way admittedly results in freedom from pain but it means that the joint becomes completely inoperable.
For implantation of earlier finger joint prosthesis, the inner collateral ligament had to be detached at the phalanx for ulnar access. The joint was then laterally luxated and a part of the palmar plate detached. The head of the first phalanx and the base of the second phalanx were then resectioned in such a way that a previously determined spacing resulted between the two phalanges. Thereupon a rectangular space extending along the center line of each phalanx was produced by a rasp in each phalanx. A shaft guide was then cemented into that rectangular space. The joint elements were then individually fitted with their anchoring bars into the shaft guide, the phalanges were bent back and the first and second joint elements were hingedly connected together by inserting a spindle into the aligned openings.
As the finger middle joint had to be laterally luxated upon implantation of the known finger joint prostheses the extensor tendon assembly, the two flexor tendons and the collateral ligaments of the joint were irritated, which resulted in later impairment of operability of the joint. In addition a large amount of bone substance was sacrificed for implantation of the finger joint prosthesis, as a shaft guide had to be inserted in the direction of the center line of the finger joints. DE 690 02 159 discloses such a prosthesis in which, in accordance with the description, a reduced loss of bone substance is to be encountered upon implantation.
A markedly improved finger joint prosthesis is known from EP 1 096 906 to the inventor Christoph Ranft. That permits a novel and improved method while avoiding the above-mentioned disadvantages. In the method described in EP 1 096 906, the finger joints prosthesis can be implanted in the assembled condition radially into a pre-prepared bore in the finger joint, which represents a marked simplification in installation and leads to slight injuries, in particular at the tendons of the finger.
On the other hand that finger joint prosthesis is of an excessively filigree structure and has a partial lack of strength and durability. DE 10 2007 008 406 discloses a radially implantable finger joint prosthesis which enjoys a higher degree of stability but which at the same time can still be easily implanted with slight injury to the patient. In accordance with DE 10 2007 008 406 that object is attained in that the first joint element has an outside hollow body having an insertion opening, the second joint element has an inside hollow body, the inside hollow body can be inserted relatively into the outside hollow body in the installation position by way of the insertion opening and a spindle can be fitted into the inside hollow body in the installation position for rotatable connection to the outside hollow body.
Although the prosthesis disclosed in DE 10 2007 008 406 has stability which is considerably improved over the state of the art, there are nonetheless certain weaknesses in anchorage, in particular in relation to loadings arising out of a distal pull. That can lead to loosening of the finger joint prosthesis, in particular on the distal anchoring bar. In that way the prosthesis can grow into place in an unwanted position. In addition growth (secondary fixing) can be delayed or be entirely absent.